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1 الصفحة 1 من 124 Bacterial toxins ( By HOZA, A.S ): TOXIGENESIS : Two types of bacterial toxins : 1. Lipopolysaccharides: are associated with the cell walls of Gram-ve bacteria. The lipopolysaccharide (LPS) component of the Gram-ve bacterial outer membrane bears the name endotoxin because of its association with the cell wall of bacteria. 2. Proteins: may be released into the extracellular environment of pathogenic bacteria. Most of the protein toxins are thought of as exotoxins, since they are "released" from the bacteria and act on host cells at a distance. A: BACTERIAL PROTEIN TOXINS ( Exotoxins ) The protein toxins are soluble proteins secreted by living bacteria during exponential growth. The production of protein toxins is specific to a particular bacterial species - (e.g. only Clostridium tetani produces tetanus toxin. - only Corynebacterium diphtheriae produces the diphtheria toxin). Usually, virulent strains of the bacterium produce the toxin (or range of toxins) while non-virulent strains do not. Toxin is the major determinant of virulence. Both Gram-positive and Gram-negative bacteria produce soluble protein toxins. Bacterial protein toxins are the most potent poisons known and may show activity at very high dilutions.

2 الصفحة 2 من 124 Exotoxins : The protein toxins resemble enzymes. Like enzymes, bacterial exotoxins are: 1. proteins. 2. denatured by heat, acid, proteolytic enzymes. 3. have a high biological activity (most act catalytically). 4. exhibit specificity of action. Bacterial protein toxins are highly specific in the substrate utilized and in their mode of action. Usually the site of damage caused by the toxin indicates the location of the substrate for that toxin. Terms such as "enterotoxin", "neurotoxin", "leukocidin" or "hemolysin" are used to indicate the target site of some welldefined protein toxins. Exotoxins : Certain protein toxins have very specific cytotoxic activity (i.e., they attack specific cells, for example, tetanus or botulinum toxins). Some (as produced by staphylococci, streptococci, clostridia, etc.) have fairly broad cytotoxic activity and cause nonspecific death of tissues (necrosis). Toxins that are phospholipases may be relatively nonspecific in their cytotoxicity. A few protein toxins cause death of the host and are known as "lethal toxins", (e.g. anthrax toxin).

3 Exotoxins : الصفحة 3 من 124 Protein toxins are strongly antigenic. In vivo, specific antibody (antitoxin) neutralizes the toxicity of these bacterial proteins. In vitro, specific antitoxin may not fully inhibit their enzymatic activity. Toxoids: are detoxified toxins which retain their antigenicity and their immunizing capacity (first discovered by Ehrlich). Exotoxins : A + B Subunit Arrangement of Protein Toxins : Many protein toxins, consist of two components: 1. Subunit A: responsible for the enzymatic activity of the toxin. 2. Subunit B: concerned with binding to a specific receptor on the host cell membrane and transferring the enzyme across the membrane. The enzymatic component is not active until it is released from the native toxin.

4 الصفحة 4 من 124 B. ENDOTOXINS: Endotoxins are: Invariably associated with Gram-ve bacteria as constituents of the outer membrane of the cell wall. ENDOTOXINS: 1. The biological activity of endotoxin is associated with the lipopolysaccharide (LPS). Toxicity is associated with the lipid component (Lipid A) and Immunogenicity (antigenicity) is associated with the polysaccharide components. 2. The cell wall antigens (O antigens) of Gram-negative bacteria are components of LPS. 3. Compared to the classic exotoxins of bacterial endotoxins are less potent and less specific in their action, since they do not act enzymatically. 4. Endotoxins are heat stable (boiling for 30 min does not destabilize endotoxin). 5. But certain powerful oxidizing agents such as, superoxide, peroxide and hypochlorite degrade them. 6. Endotoxins, although strongly antigenic, cannot be converted to toxoids. Lipopolysaccharides: Are complex amphiphilic molecules with a mw of about 10kDa, that vary widely in chemical composition both between and among bacterial species. LPS consists of three components: 1. Lipid A. 2. Core polysaccharide. 3. O polysaccharide.

5 Lipid A : الصفحة 5 من 124 Lipid A consists of a phosphorylated N-acetylglucosamine (NAG) dimer with 6 or 7 fatty acids (FA) attached. The structure of Lipid A is highly conserved among Gram-ve bacteria. Among Enterobacteriaceae Lipid A is virtually constant. Core (R) polysaccharide: Is attached to the 6 position of one N-acetylglucosamine ( NAG). Is common to all members of a bacterial genus with minor variations (e.g. Salmonella). Salmonella, Shigella and Escherichia have similar but not identical cores. O polysaccharide (also referred to as the O antigen or O side chain): Is attached to the core polysaccharide. Consists of repeating oligosaccharide subunits made up of 3-5 sugars. A major antigenic determinant (antibody-combining site) of the Gram-negative cell wall resides in the O polysaccharide. LPS and virulence of Gram-negative bacteria : Endotoxins are toxic to most mammals. They are strong antigens but they seldom elicit immune responses which give full protection to the animal against secondary challenge with the endotoxin. Cannot be toxoided. Endotoxins released from multiplying or disintegrating bacteria significantly contribute to the symptoms of Gram-ve bacteremia and septicemia important pathogenic factors in Gram-ve infections.

6 الصفحة 6 من 124 Regardless of the bacterial source : All endotoxins produce the same range of biological effects in the animal host. Injection of living or killed Gram-ve cells, or purified LPS, into experimental animals wide spectrum of nonspecific pathophysiological reactions. The role of Lipid A : Physiological activities of endotoxins- mediated mainly by the Lipid A component of LPS. Lipid A is the toxic component of LPS. Lipid A is known to react at the surfaces of macrophages release cytokines that mediate the pathophysiological response to endotoxin. The role of the O polysaccharide : Although nontoxic, the polysaccharide side chain (O antigen) of LPS may act as a determinant of virulence in Gram-ve bacteria. polysaccharide is responsible for the property of "smoothness" of bacterial cells, which may contribute to their resistance to phagocytic engulfment. O-polysaccharide is antigenic, and the usual basis for antigenic variation in Gram-ve bacteria rests in differences in their O polysaccharides.

7 الصفحة 7 من 124 Pathogenicity Islands (PAI): Are a distinct class of genomic islands which are acquired by horizontal gene transfer. Incorporated in the genome of pathogenic bacteria usually absent from non-pathogenic organisms of the same or closely related species. They occupy relatively large genomic regions ranging from kb. encode genes which contribute to virulence of the pathogen. One species of bacteria may have more than one pathogenicity island. o For example, in Salmonella, five pathogenicity islands have been identified. Found mainly in Gram-ve bacteria, but have been shown in a few Gram-positives. May be located on the bacterial chromosome or may be a part of a plasmid. Play a vital role in the virulence of bacterial pathogens of humans, animals and plants. May also represent a mechanism that contributes to the appearance of new pathogens. Genomic islands play an important role in microbial evolution. Knowledge about PAI, their structure, their mobility, and the pathogenicity factors they encode is helpful: A- In gaining a better understanding of bacterial evolution and interactions of pathogens with eucaryotic host cells. B - Also may have important practical implications such as providing delivery systems for vaccination and tools for the development of new strategies for therapy of bacterial infections.

8 الصفحة 8 من 124 Resistance to disease: Immune system : Two intrinsic systems: Innate (nonspecific) defense system. Adaptive (specific) defense system. Functional system rather than organ system. Innate and adaptive defenses intertwined. Release and recognize many of same defensive molecules. Innate defenses do have specific pathways for certain substances. Innate responses release proteins that alert cells of adaptive system to foreign molecules. Innate defense system has two lines of defense : First - external body membranes (skin and mucosae). Second - antimicrobial proteins, phagocytes, and other cells Inhibit spread of invaders. Inflammation most important mechanism. Adaptive defense system : Third line of defense attacks particular foreign substances,takes longer to react than innate system.

16 الصفحة 16 من 124 The Normal Bacterial Flora of Humans In healthy human, there are many of bacteria are consistently associated with the body 1- Body surfaces. 2- Mucus membranes. skin and mucous membranes, are always contact with environmental organisms and become colonized by various bacterial species. 3- Intestinal linings of humans. The mixture of organisms or the bacteria and other microbes that are consistently associated with human and found at any site are called the normal flora, or the "indigenous microbiota". These bacteria have a symbiotic interactions with their hosts. The normal flora of humans consists of a few fungi, but bacteria are the most numerous and obvious microbial components of the normal flora The internal tissues, e.g. blood, brain, muscle, etc., are normally free of microorganisms. The normal flora derive from their host a steady supply of nutrients, a stable environment, protection and transport. The host obtains from the normal flora certain nutritional and digestive benefits, stimulation of the development and activity of immune system, and protection against colonization and infection by pathogenic bacteria.

17 الصفحة 17 من 124 Endogenous diseases : ( En do geno us ) Diseases that are produced by the normal flora in their host. Most endogenous bacterial diseases are opportunistic infections, meaning that the organism must be given a special opportunity of weakness in the host defenses in order to infect. Harmful Effects of the Normal Flora: The overall harmful effects of microbes are summarized below: 1- Bacterial synergism: Between a member of the normal flora and a potential pathogen helps in establishment of infection. There are examples of a member of the normal flora supplying a vitamin or some other growth factor that a pathogen needs in order to grow. This is called cross-feeding between microbes 2- Competition: For nutrients Bacteria in the gastrointestinal tract may get to some of our utilizable nutrients before we are able to absorb them. 3- Induction of a low grade toxemia: Minute amounts of bacterial toxins (e.g. endotoxin) may be found in the circulation. Of course, it is these small amounts of bacterial antigen that stimulate the formation of natural antibodies. 4- The normal flora may be agents of disease: Members of the normal flora may cause endogenous disease if they reach a site or tissue where they cannot be restricted or tolerated by the host defenses. Many of the normal flora are potential pathogens, and if they gain access to a compromised tissue from which they can invade, disease may result.

18 الصفحة 18 من Transfer to susceptible hosts : Some pathogens of humans that are members of the normal flora may also rely on their host for transfer to other individuals where they can produce disease. This includes the pathogens that colonize the upper respiratory tract such as : Neisseria meningitidis, Streptococcus pneumoniae, Haemophilus influenzae and Staphylococcus aureus, And potential pathogens such as: E. coli, Salmonella or Clostridium in the gastrointestinal tract.

23 الصفحة 23 من 124 External and Internal Barriers Physical and Chemical Barriers : Physical and chemical barriers protect against a wide variety of invaders. Few pathogens can pass through the tough layer of dead skin cells that surrounds the body. Tears and saliva contain enzymes that destroy or disable many pathogens. Mucus secreted by mucous membranes carries trapped pathogens to other areas of the body for disposal. Cilia sweep mucus and pathogens to the throat, where they can be swallowed or coughed out. Gastric juice in the stomach destroys many pathogens that enter the body through the nose and mouth. Defense Strategies of the Immune System: The immune system has two major defense strategies: 1. The inflammatory response is general, or nonspecific; it works against all types of pathogens. 2. Specific defenses work against particular pathogens.

29 الصفحة 29 من 124 Second line of defense : Once beyond the protective outer barrier of the body, the invading microbes will encounter a series of nonspecific cellular and chemical defense mechanisms. Mechanisms: Inflammation a series of events that removes or contain the offending agent and repair the damage. Chemotaxis movement of cells toward a chemical influence (chemokines or chemotatic agents). Phagocytosis process in which cell ingest foreign particulate matter e.g. microbes. Many are carried out by the white blood cells in blood. Phagocytosis is the ingestion of microorganisms or other matter by a cell. Many white blood cells engulf invasive microorganisms by the process of phagocytosis. The steps in phagocytosis are: 1. Chemotaxis is the process by which phagocytes are attracted to microorganisms. 2. Attachment: The phagocyte then adheres to the microbial cell. This adherence may be facilitated by opsonization coating the microbe with plasma proteins. 3. Ingestion: Pseudopods of phagocytes engulf the microorganism and enclose it in a phagosome to complete ingestion. 4. Digestion: Lysosomes fuse with the phagosome to form a digestive vacuole. The microbe is killed and digested.

32 الصفحة 32 من 124 Streptococci : The streptococci are : Gram positive spherical bacteria characteristically arranged in pairs or chains. Lengths of chains are conditioned by environmental factors. Growth characteristics : Streptococci grow in solid media, supplemented with blood, as discoid colonies, usually one to two mm in diameter. Some streptococci : (β hemolytic) can lyse blood cells and cause a complete clearing of blood all around the colonies. Other strains cause no change in blood agar ( γ or nonhemolytic). Other reduce hemoglobin and cause a greenish discolouration of agar (α hemolytic). Hemolytic reactions on culture media: In beta hemolytic reaction red blood cells are completely lysed. In alpha hemolytic reaction red blood cells are not completely lysed but colonies are surrounded by greenish discolouration of agar due to streptococcal action on haemoglobin. Nonhemolytic or gamma hemolytic streptococci have no effect on blood agar.

33 الصفحة 33 من 124 Lancefield classification: An important method of distinguishing between pyogenic streptococci is the serological classification pioneered by the American bacteriologist Rebecca Lancefield She detected different versions of the major cell wall polysaccharide among the pyogenic streptococci. The polysaccharide is referred to as the group polysaccharide identifies a number of different groups labeled by capital letters ( Lancefield groups A,B,C. etc. ). Lancefield subdivided streptococci based on cell wall antigen (antigen C). This carbohydrate forms the basis of serologic grouping. For group A streptococci, antigen C is rhamnose-nacetylglucosamine. For group B, streptococci, antigen C is rhamnose-glucosamine polysaccharide etc. Group A contains most of the streptococci that cause infection in humans. Groups B to O are less pathogenic and are often present without causing disease. Most are hemolytic; of those, the beta subgroup is the most likely to be the cause of infection. Complexity Simple Carbohydrates monosaccharides Complex Carbohydrates disaccharides, oligosaccharides & polysaccharides

34 الصفحة 34 من 124 Streptococcus pyogenes (Group A streptococcus): Is a Gram-positive, nonmotile, nonsporeforming coccus that occurs in chains or in pairs of cells. Individual cells are round-to-ovoid cocci, micrometer in diameter. Streptococci divide in one plane and thus occur in pairs or (especially in liquid media or clinical material) in chains of varying lengths. The metabolism of Str. pyogenes is fermentative; the organism is a catalase-negative aerotolerant anaerobe (facultative anaerobe), and requires enriched medium containing blood in order to grow. Group A streptococci typically have a capsule composed of hyaluronic acid and exhibit beta (clear) hemolysis on blood agar.

36 الصفحة 36 من 124 The cell surface of Streptococcus pyogenes accounts for many of the bacterium's determinants of virulence, especially those concerned with colonization and evasion of phagocytosis and the host immune responses. The surface of Streptococcus pyogenes is incredibly complex and chemically-diverse. Antigenic components include capsular polysaccharide (C-substance), cell wall peptidoglycan and lipoteichoic acid (LTA), and a variety of surface proteins, including M protein, fimbrial proteins, fibronectin-binding proteins, (e.g. Protein F) and cell-bound streptokinase.

37 الصفحة 37 من 124 Clinical features: 1. Streptococcus pyogenes is one of the most frequent pathogens of humans. 2. It is estimated that between 5-15% of normal individuals harbor the bacterium, usually in the respiratory tract, without signs of disease. 3. As normal flora, Str. pyogenes can infect when defenses are compromised or when the organisms are able to penetrate the constitutive defenses. 4. When the bacteria are introduced or transmitted to vulnerable tissues, a variety of types of suppurative infections can occur. 5. In the last century, infections by Str. pyogenes claimed many lives especially since the organism was the most important cause of puerperal fever (sepsis after childbirth). 6. Scarlet fever was formerly a severe complication of streptococcal infection, but now, because of antibiotic therapy, it is little more than streptococcal pharyngitis accompanied by rash. 7. Similarly, erysipelas (a form of cellulitis accompanied by fever and systemic toxicity) is less common today. 8. However, there has been a recent increase in variety, severity and sequelae of Streptococcus pyogenes infections, and a resurgence of severe invasive infections, prompting descriptions of "flesh eating bacteria" in the news media. 9. A complete explanation for the decline and resurgence is not known. Today, the pathogen is of major concern because of the occasional cases of rapidly progressive disease and because of the small risk of serious sequelae in untreated infections.

38 الصفحة 38 من These diseases remain a major worldwide health concern, and effort is being directed toward clarifying the risk and mechanisms of these sequelae and identifying rheumatogenic and nephritogenic strains of streptococci. 11. Acute Streptococcus pyogenes infections may present as pharyngitis (strep throat), scarlet fever (rash), impetigo (infection of the superficial layers of the skin) or cellulitis (infection of the deep layers of the skin). Invasive, toxigenic infections can result in necrotizing fasciitis, myositis and streptococcal toxic shock syndrome. 12. Patients may also develop immune-mediated post-streptococcal sequelae, such as acute rheumatic fever and acute glomerulonephritis, following acute infections caused by Streptococcus pyogenes. Treatment and prevention: Penicillin is still uniformly effective in treatment of Group A streptococcal disease. It is important to identify and treat Group A streptococcal infections in order to prevent sequelae. No effective vaccine has been produced, but specific M-protein vaccines are being tested. (sequelae : a condition that is the consequence of a previous disease or injury. " the long-term sequelae of infection " ).

39 Staphylococci: الصفحة 39 من 124 Staphylococci are Gram-positive spherical cells, usually arranged in irregular clusters (grape-like). They grow on many types of media, are active metabolically, fermenting carbohydrates and producing pigments that vary from white to deep yellow. Some are members of normal flora of skin and mucous membranes of humans, others cause suppuration, abscess formation, toxin mediated diseases, and fatal septicemia Classification: Staphylococcus genus has about 30 species. The main species of clinical importance are: Staphylococcus aureus Staphylococcus epidermidis Staphylococcus saprophyticus Staphylococcus aureus is coagulase-positive, which differentiates it from the other species. Staphylococcus epidermidis and Staphylococcus saprophyticus are coagulase negative. Staphylococcus saprophyticus is the cause of urinary tract infection in young women.

40 الصفحة 40 من 124 Staphylococci culture: Staphylococci grow on most bacteriologic media under aerobic or microaerophilic condition at 37 C. The colonies on solid media are round, smooth, raised, glistening. Staphylococcus aureus form gray to deep golden yellow colonies. Staphylococcus epidermidis form gray to white colonies. Staphylococci : Produce catalase (which differentiates them from the streptococci). Ferment many carbohydrates. Are resistant to drying, heat and 7.5% sodium chloride. Are sensitive to many antimicrobial drugs. Staphylococcus aureus: 1. Staphylococcus aureus is a major pathogen for humans. 2. Many people are asymptomatic carriers; they have staphylococci on the skin and in the throat (opportunistic pathogen). 3. As a nosocomial pathogen, Staph. aureus has been a cause of morbidity and mortality. 4. In hospitals, the areas at risk for severe staphylococcal infections are the newborns nursery, intensive care units, operating rooms and cancer chemotherapy wards.

42 الصفحة 42 من 124 Virulence factors of Staphylococci: Staphylococci can produce diseases through their ability to multiply in tissues and through production of many extracellular and cellular substances. Toxins Hemolysins (cytolytic) Hemolysins (cytolytic): α it is potent hemolysin, degrades red blood cells of rabbits. β it degrades sphingomyelin and is toxic for red blood cells of sheep. γ it lyses red blood cells of humans. δ it disrupts biologic membranes and may have a detergent role. Leukocidin (Panton-Valentine toxin): This toxin has two components. It can kill white blood cell of humans and rabbits; two components act together to form pores and they increase cation permeability. Panton-Valentine leukocidin (PVL) is a cytotoxin one of the β- pore-forming toxins. The presence of PVL is associated with increased virulence of certain strains (isolates) of Staphylococcus aureus. It is present in the majority of community-associated Methicillinresistant Staphylococcus aureus (CA-MRSA) isolates studied and is the cause of necrotic lesions involving the skin or mucosa, including necrotic hemorrhagic pneumonia. PVL creates pores in the membranes of infected cells. PVL is produced from the genetic material of a bacteriophage that infects Staphylococcus aureus, making it more virulent. It was initially discovered by Van develde in 1894 due to its ability to lyse leukocytes. It was named after Sir Philip Noel Panton and Francis Valentine when they associated it with soft tissue infections in 1932.

43 الصفحة 43 من 124 Exfoliative toxins: Are two distinct proteins with same molecular weight, they cause generalized desquamation on staphylococcal scalded skin syndrome. SSSS is typical in (superantigen). newborns and in infants younger than 1 year ==================================================== Toxic Shock Syndrome toxin TSST-1 (superantigen). It binds to MHC class II molecules yielding T cell stimulation which promotes the manifestations of toxic shock syndrome. This syndrome is associated with fever, shock and multisystem involvement including desquamative skin rash. ==================================================== Enterotoxins there are multiple enterotoxins; like TSST-1 they are superantigens. Enterotoxins are heat-stable and resistant to action of enzymes. Enterotoxins cause food poisoning, they are produced when S. aureus grows in fatty foods; ingestion results in vomiting and diarrhea. Emetic effect probably is the result of central nervous system stimulation (vomiting centre) when the toxin acts on neuron receptors in intestinal tract.

44 Enzymes : الصفحة 44 من 124 Coagulase this enzyme clots oxalated or citrated plasma. Coagulase binds to prothrombin; together they become enzymatically active and initiate fibrin polymerization. Coagulase may deposit fibrin on surface of staphylococci altering their ingestion by phagocytic cells. ==================================================== Catalase : converts hydrogen peroxide into water and oxygen. Catalase test differentiates staphylococci (positive) from streptococci (negative). ==================================================== Hyaluronidase it is spreading factor. Staphylokinase fibrinolitic enzyme, but acting much more slowly than Streptokinase. Proteinases and lipases they act on proteinic and lipidic components.

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46 الصفحة 46 من 124 Diagnostic laboratory tests: Samples: A throat swab, a samples of pus, blood, tracheal aspirate, or spinal fluid may be used (in according with the different localization of process). Direct examination : with a direct microscopic examination is not possible to distinguish saprophyticus from pathogenic organisms; culture and appropriate identification techniques must confirm this report. Isolation: Specimens planted on blood agar plates develop the typical colonies in 18 hours at 37 C, but hemolysis and pigment production are optimal at room temperature. Identification:Isolated colonies on blood agar medium are planted on Mannitol salt agar. This medium conteins: mannitol, 7,5% of Na Cl and a ph indicator. Staph. aureus ferments mannitol : The salt (7,5% NaCl) inhibits the most other normal flora but not staphylococci. Mannitol salt agar is medium used to screen S. aureus from S. epidermidis

47 الصفحة 47 من 124 Biochemical identification: Coagulate test : two different coagulase tests can be performed: A tube test for free coagulase, and a slide test for bound coagulase or clumbing factor. The slide coagulase test is performed by making uniform suspension of growth, adding 1 drop of plasma and observing for clumping within 10s (this test may be used as rapid screening technique to identify S. aureus). Catalase staphylococci produce catalase, which forms water and oxygen from hydrogen peroxide. The test differentiates staphylococci, which are positive, from streptococci, which are negative. Antimicrobial treatment: Staphylococcal isolates should be tested for antimicrobial susceptibility because S. aureus has developed resistance to all antibiotic classes available for clinical use. Resistance to Penicillin: The most common mechanism of Saph. aureus resistance to β-lactam involves penicillinase an enzyme that hydrolyzes penicillin into inactive penicilloid acid. Penicillinase-producing strains emerged rapidly after penicillin introduction in the mid 1940s and became prevalent in hospitals and in communities. (Penicillin G resistant Saph. aureus strains, producing penicillinase, now constitute about 90% of isolates in communities). In the late 1950 a new penicillinase-resistant penicillin called Methicilllin was created.

48 الصفحة 48 من 124 Hospital acquired methicillin- resistant Staphylococcus aureus : Resistance to Methicillin: The first penicillinase-stable β-lactam such as methicillin and the isoxazolyl penicillins became available in the late 1950s. The first MRSA was described at aboud the same time. The prevalence of MRSA in hospital has increased rapidly in the last periods (more than 60% in hospital centers with great geographic variations). Mechanism of Methicillin resistance: The mechanisms is mediated by a new acquired penicillin binding protein 2A ( PBP2A), encoded by meca gene. ( The meca gene is a gene found in bacterial cells. The meca gene allows a bacterium to be resistant to antibiotics such as methicillin, penicillin and other penicillin-like antibiotics.) PBP2A A rapid immunochromatographic qualitative assay for the detection of penicillin binding protein 2a (PBP2a) direct from S. aureus culture isolates as an aid in detecting methicillin-resistant Staphylococcus aureus (MRSA). Because of its low β-lactam affinity, PBP2A can complete cell wall synthesis. MRSA Methicillin resistant Staphylococcus aureus : Methicillin resistant Staph. aureus are bacteria which have developed resistance to beta-lactam antibiotics those which target cell wall synthesis of bacteria. Such antibiotics includes the penicillin family of drugs. Their resistance is conferred by the gene meca, which encodes for the protein penicillin-binding protein 2a (PBP2).

49 الصفحة 49 من 124 Alternative treatments for MRSA: Treatment of infections with hospital multiresistant MRSA appears problematic. Vancomycin is the first choice in these situation. Members of aminoglycosides associated with vancomycin increase their bactericidal activity. Quinupristin-Dalfopristin is a combination of a streptogramin B and A, this association is sinergic and shows a large activity against MRSA strains. Linezolid belongs to a new oxazolidinone family of molecules, it is active against all multiresistant Gram positive pathogens. Daptomycin is a cyclic lipopeptide antibiotic active against MRSA.

51 الصفحة 51 من 124 Pathogenic Enterobacteriaceae are often classified into three groups 1- Coliforms, rapidly ferment lactose, part of the normal microbiota, may be opportunistic pathogens. Examples: Escherichia, Klebsiella, Enterobacter, Citrobacter, Serratia. Presence of coliforms in water is indicate of impure water and of poor sewage treatment (i.e. one of the indicators of fecal pollution of water: E. coli) 2- Non coliform opportunists, do not ferment lactose. 3- True pathogens. Escherichia coli : Motile. Lactose-fermenting mucoid colonies on macconkey. Some strains are hemolytic on blood agar. Produce indole. aerobic or anaerobic. can grow in the presence or absence of O 2. typically oxidase-negative. most strains ferment lactose & glucose with the production of acid and gas. The most common and important of the coliforms found in 100% of human intestines, live in the intestinal tracts of animals in health and disease. Gastroenteritis is the most common disease associated with E. coli (enteropathogenic, enterotoxigenic and enteroinvasive strains). Often mediated by exotoxins that produce the symptoms associated with gastroenteritis.

54 Klebsiella: الصفحة 54 من 124 Usually found in GI tract. Four major species. K. pneumoniae is mostly commonly isolated species. Possesses a polysaccharide capsule, which protects against phagocytosis and antibiotics AND makes the colonies moist and mucoid. Has a distinctive yeasty odor. Frequent cause of nosocomial pneumonia. Significant biochemical reactions : Lactose positive. Most are urease positive. Non-motile. Capsulated. large gram-negative rods. Found in the digestive and respiratory systems of humans and animals. Can cause opportunistic infections- hospital acquired (nosocomial). No water borne disease ever associated with Klebsiella in drinking water.

55 الصفحة 55 من 124 Main species of medical importancce: K. pneumonia ( Pneumoniae ). K. rhinoscleromatis ( rhinoscleroma ). K. ozenae ( ozena ). K. pneumoniae: most commonly isolated pathogenic species- meningitisbacteremia. It is found as a commensal in the intestinal tract, and also found in moist environment in hospitals. It is an important nosocomial pathogen. It causes: Pneumonia- Urinary tract infection- Septicemia and meningitis (especially in neonates) Wound infection and peritonitis. Treatment: Based on sensitivity testing.

56 Serratia : الصفحة 56 من 124 Produce a red pigment when grown at room temperature Can grow on catheters, in saline solutions, and other hospital supplies. Doesn t ferment lactose on MacConkey agar media. Can cause life-threatening opportunistic infections in the urinary and respiratory tracts of immunocompromised patients. Difficult to treat due to resistance to various antimicrobial drugs. Seven species, but S. marcescens is the only one clinically important. Frequently found in nosocomial infections of urinary or respiratory tracts. Implicated in bacteremic outbreaks in nurseries, cardiac surgery, and burn units. Fairly resistant to antibiotics. Species : Serratia marcescens. Serratia rubidaea. Serratia liquifaciens. Serratia odorifera. Major characteristics: o Ferments lactose slowly. o Produce characteristic pink pigment, especially when cultures are left at room temperature. o Serratia can be distinguished from other genera belonging to Enterobacteriaceae by its production of three special enzymes: DNase, lipase, and gelatinase. o Some strains are hemolytic.

59 Pathogenesis: الصفحة 59 من 124 o The normal habitat of Enterobacter spp. Is probably soil and water, but the organisms occasionally found in human faeces and the respiratory tract. o Enterobacter spp. Are much less important cause of hospital infection than Klebsiella spp. o Most infection are of the urinary tract. o Although members of genus are an important cause of bacteraemia in some hospitals. o Strains may produce a haemolysin resembling the alph haemolysin produced by strains of Esch.coli. o An outer membrane protein termed OmpX, may be a pathogenic Treatment : factor for some strains of Enterobacter. o Highly resistant to penicillins and many cephalosporins ( strains with cephalosporinase ). o Resistant to tetracycilnes and streptomycin. o Sensitive to gentamicin. polymyxins.

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Small living organism Not visible to the naked eye Must be viewed under a microscope Found everywhere in the environment, including on and in the human body Many Microorganisms are part of normal flora

OCR (A) Biology A-level Topic 4.1: Communicable diseases, disease prevention and the immune system Notes Pathogens Bacteria and viruses are the main disease causing pathogens in humans. Even though they